This invention relates generally to the field of kits for heating and cooling systems, and more particularly to a control box kit for twinning operation of a heating, cooling, or heat pump system.
In a typical heating or cooling system, a controller or control circuit connects a thermostat to a heating or cooling device. The thermostat initiates a demand for heating or cooling which signals the heating/cooling device to turn on and off. The controller or control circuit receives the signal from the thermostat and controls the action of the heating/cooling device. Such heating/cooling devices include furnaces, air conditioners, combined furnace/air conditioner systems which share an air handling system, and heat pumps.
Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop to a relatively cooler side of the circulation loop. Compression of the refrigerant occurs at the hotter side of the loop, where a compressor raises the temperature of the refrigerant. Evaporation of the refrigerant occurs at the cooler side of the loop, where the refrigerant is allowed to expand, thus resulting in a temperature drop. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor mediums, respectively, to make use of the outdoor mediums as either a thermal energy source or a thermal energy sink. In the case of an air to water heat pump, outdoor air is used as a thermal energy source while water is used as a thermal energy sink.
The process is reversible, so the heat pump can be used for either heating or cooling. Residential heating and cooling units are bidirectional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
In many cases, it is more cost effective or practical to install two residential-type units of moderate capacity than one commercial-type unit of large capacity. Twinned units typically operate off one thermostat. In a one-stage twinned system, both units turn on and off simultaneously. In a two-stage or multi-stage twinned system, the two units cycle separately in a prescribed manner.
Under certain operating conditions, frost builds up on a coil of the heat pump. Coil frosting results in lower coil efficiency while affecting the overall performance (heating capacity and coefficient of performance (COP)) of the unit. From time to time, the coil must be defrosted to improve the unit efficiency. In a twinned system, defrosting both units simultaneously is inefficient.
U.S. Pat. No. 5,316,073 discloses a twinning control for use on HVAC systems which is based on a microprocessor with LED indicators. Although such a control system works, the relative low volume of twinned fan coil installations does not justify developing a microprocessor based system. Making individual hand connections between thermostats, relays, fan coils, and heating/cooling units is cumbersome and time consuming.
Briefly stated, a control kit for twinning fan coils in a heat pump or AC installation includes electro-mechanical isolation relays and auxiliary limit switches. Existing fan coil transformers are disconnected to avoid component failure due to high voltage wiring variations. A single transformer is connected to operate both systems. Isolation relays for the reversing valves and supplemental electric heaters in a heat pump system allow both heat pumps to have independent defrost cycles. The accessory transformer and isolation relays are packaged and pre-wired in a control box for easy connection in the field. The accessory limit switches are integral safety devices to assure safe operation in the case of improper fan coil operation in heating mode.
According to an embodiment of the invention, a control box kit for twinning first and second units of an HVAC system, wherein the first and second units are either first and second air conditioning units or first and second heat pump units, and the system includes first and second fan coils and a thermostat, includes a control box; a single transformer which provides low voltage power to both of the units, the transformer disposed inside the control box; and connection wires inside the control box pre-wired for a preselected twinned-unit operating configuration such that an installer makes all field connections for the preselected twinned-unit operating configuration between the thermostat, the first and second fan coils, and the first and second units to the connection wires.
According to a feature of the invention, the preselected twinned-unit operating configurations include (a) single-stage operation of the first and second heat pump units; (b) multi-stage operation of the first and second heat pump units; (c) single-stage operation of the first and second air conditioning units; and (d) multi-stage operation of the first and second air conditioning units.
According to a feature of the invention, the control box kit further includes a terminal strip attached to the control box, the terminal strip having a plurality of terminal posts extending from outside the control box to inside the control box, wherein the connection wires are connected to the terminal posts inside the control box, and the field connections are made at the terminal strip outside the control box.